This invention is related to a device for purifying exhaust gases from a combustion engine according to the precharacterizing part of enclosed claim 1. Besides, the invention is related to a method for exhaust gas purification and use of the device for exhaust gas purification in particular at a diesel engine.
It is known that EGR (Exhaust Gas Recirculation) is an advantageous purification method for reducing the proportion of hazardous exhaust gases, in particular nitrogen oxide (NOx). In an EGR-system, a part of the exhaust gases from the engine are recirculated to an air intake thereof.
It is also known to use exhaust gas purification arrangements comprising at least one converter unit for converting constituents of the exhaust gases to less environmentally hazardous substances. According to the present state of the art, such converter units comprise, generally, catalysts for achieving a catalytic conversion of constituents in the exhaust gases to less environmentally hazardous substances. Thus, by means of such catalysts, carbon monoxide and hydro carbons may be converted to carbon dioxide and water. This presupposes that the exhaust gases contain a certain amount of oxygen. For this purpose an oxygen measuring unit is generally used in the exhaust gas flow from the engine and this unit delivers output signals, on basis of which the operation of the engine is controlled to achieve the required oxygen contents. Furthermore, also nitrogen oxides may be converted to neutral nitrogen by means of such catalysts. An excess of oxygen in the exhaust gases would give rise to cessation of the reduction of nitrogen oxides whereas a deficiency with respect to oxygen would counteract conversion of the other constituents mentioned above in the exhaust gases. An optimal regulation of the fuel system may, however, cause a decrease of all above mentioned, hazardous constituents. By using EGR technique, a further reduction of nitrogen oxides may be achieved.
In addition, there exists the problem, in particular in diesel engines, that they generate a substantial amount of particulate constituents. Within the framework of the expression particulate constituents there are included both particles as such, for instance soot, and organic residues (denominated SOF) which emanate from fuel and oil. It is known to use filters of various types to liberate the exhaust gases from such particulate constituents. It is also known to design such filters as regenerating, i.e. that they may be restored without exchange. Such regeneration is according to the prior art achieved by heating the filters to a required degree so that combustion of the particulate constituents occurs. The energy requirement for such combustion is very large, for what reason one has had, according to the prior art, to immobilise the filter, either still coupled to the engine or removed therefrom so that by connection of a heating element to an electric power network the required heating may occur. Thus, this necessitates an, interruption of operation. Another technique (U.S. Pat. No. 5,207,734 and JP 8338320) to achieve regeneration of a filter in an EGR recirculation conduit is to use a catalyst upstreams of the filter to provide for a heat addition to the filter from the catalyst. However, this results in deficient filter regeneration, in particular when the recirculated exhaust gas amount is small as it is under some engine operating conditions.
The object of the present invention is to develop the prior art for the purpose of achieving efficient filter regeneration and efficient purification with regard to NOx, carbon monoxide, hydro carbons, particles etc.
This object is achieved by the features of enclosed claim 1.
The present invention is, accordingly, based upon the idea to arrange the filter so that heat in the exhaust gases and in addition the heat which occurs as a consequence of the conversion in the converter unit may be transversely transported from the converter unit to the filter so that the conditions for regeneration of the filter are substantially improved. It is pointed out that in EGR systems, the recirculated exhaust gas volume varies depending upon the operational conditions of the engine. During some conditions small volumes per time unit pass the filter. The heating requirement of the filter for regeneration may then not be satisfied by the heat in the exhaust gases flowing through the filter only. According to the invention it is possible to reach such high temperatures of the filter that only a comparatively small heat addition, if any, is required in order to achieve, also under difficult operational conditions, the necessary filter regeneration, i.e. combustion of particulate constituents deposited in and on the filter. More specifically, conditions are in this way created to bring the filter to the necessary regeneration temperatures by means of one or more heating elements having a relatively low effect. The energy supply to such heating elements does not become higher than making electric systems provided on e.g. vehicles capable of producing the energy generation.
Further preferable embodiments of the invention are dealt with in the rest of the claims and in the following description.
The method according to the invention and use of the device are recited in the enclosed claims.